1. Field of the Invention
This invention relates to fuses and particularly relates to fuses which protect equipment from large voltage transients upon blowing.
2. Review of the Prior Art
Fuses are widely used to protect electrical equipment, motors, instruments, electronic equipment, and the like from excessive currents resulting from sudden overloads or accidental shorts, so that the fuses must blow very quickly in order to open the circuit and prevent further damage. The sudden drop in current which occurs when a fuse blows causes a large voltage transient in a circuit having substantial inductance, such as those containing motors, transformers, and/or relays. Such large voltage transients which accompany the blowing of a fuse often cause serious damage to equipment in the circuit.
Closing or opening a switch, as with a circuit breaker, must also be done rapidly in many situations. Development of high voltage transients, arcing, contact resistance, contact bounce, and contact capacitance are some of the problems that are encountered in practical switching.
U.S. Pat. No. 3,898,602 discloses a totally enclosed, non-vented expulsion fuse accommodating universal fuse links and providing non-violent expulsion fuse operation and time-current characteristics of the overhead-cutout-type operation in a completely underground power distribution system.
For cutting off currents of high intensity, greater than 1000 amperes and up to several million amperes, with very high excess cut-off voltages greater than 10,000 volts, very rapid opening electrical current breakers are provided as described in U.S. Pat. No. 3,848,099. Such a breaker comprises two coaxial conductor elements which are connected by a circular plate having the open end of an annular cup on one side and a gaseous detonating mixture within a closed chamber on the other side, whereby a heavy current detonates the mixture and drives a cutout annular portion of the plate into the cup, thereby breaking the current passing through the plate.
U.S. Pat. No. 3,728,657 describes an electric fuse having a current-sensing section and an arc-quenching section which are mounted within a non-conductive casing having conductive metal ends. The former responds to potentially hurtful overloads and to short circuits to initiate circuit-opening action. The latter immediately takes over and instantly completes the circuit-opening action. Because it does not have to complete the circuit-opening action, the circuit-sensing section can be small. The arc-quenching section includes lead-in wires of a pyrophoric metal which are connected to opposed ends of the current-sensing section and which are brought together in close proximity before being connected to the metal ends of the fuse. When the current-sensing section blows, an arc is established between the two wires which quickly burn away towards the metal ends of the electric fuse, thereby extinguishing the arc which cannot sustain itself over the increasing distance.
The problem of electric arc formation as switches are opened and closed in high-current electrical circuits, serving a plurality of electrical cells and carrying 300,000-500,000 amperes, is met according to U.S. Pat. No. 3,542,987 by using flexible, bowed straps of conductive material which are resiliently mounted and connected to secondary contact surfaces so that electrical arc formation is directed and absorbed by the secondary contacts as the switch is opened and closed.
For example, an elastomeric material (e.g., silicone rubber) may be homogeneously and highly loaded with silicon carbide powder and a lesser amount of a group IVB element-carbide powder, as described in U.S. Pat. No. 4,331,948. The material is placed across a semiconductor junction or between electrodes of a device to be protected so that when high-voltage surges appear across the junction or device, the material breaks down (conducts) before the junction or device is destroyed. Unlike a fuse, it repeatedly recovers.
It has also long been known to utilize non-metallic current-carrying conductors which are enclosed in insulating jackets or the like, and it has further been recognized that certain types of such non-metallic conductors display distributed resistance characteristics. Graphitized polyacrylonitrile (PAN) has been utilized in composite auto ignition cables, for example, such as those described in U.S. Pat. No. 4,369,423.
The yarns for such graphite filaments are produced by controlled pyrolysis of organic precursor fibers, and preferably by pyrolysis of PAN yarns or fibers. Typically, the PAN yarn is initially heated in an oxidizing atmosphere at temperatures of the order of 200.degree.-250.degree. C., subsequently in a non-oxidizing atmosphere to 1000.degree. C. or above to carbonize the fibers comprising the yarn, and the thereafter to temperatures of the order of 1000.degree. to 2000.degree. C. to graphitize the materials and produce higher modular fibers.
Several patents, such as U.S. Pat. Nos. 3,673,035 to Whitney and 4,069,297 to Saito and British Pat. Nos. 1,257,481 to Rolls-Royce and 1,344,374 to Sosedov et al, disclose minimum carbonization temperatures on the order of 500.degree. C. as a step in processes for improving the physical properties of the raw fiber, such as tenacity or Young's modulus. Other patents teach carbonization temperatures on the order of 700.degree. C., such as U.S. Pat. Nos. 3,285,696 to Tsunoda, 3,497,318 to Nos, 3,533,743 to Prescott, 3,607,059 to Joo, 3,988,426 to Ogawa et al., 4,237,108 to Fukuhara et al. and 4,237,109 to Hiramatsu et al., and British Pat. No. 1,241,937 to Monsanto.
The conductive properties of carbon filaments have been exploited, for example, in making conductive moldable materials as discussed in U.S. Pat. No. 3,406,126 to Litant. It has also been proposed to employ low resistivity, pyrolyzed carbon fibers as light weight electrical conductors. Accordingly, changes in resistivity with pyrolyzation temperature, T.sub.p, have been the subject of experimentation in the field.
The electrical conductivity of oxidized polyacrylonitrile (PAN) fiber has been studied as a function of heat treatment temperature between 710.degree. F. and 950.degree. K. See N. R. Lerner, "Electrical Conductivity and Electron-Spin Resonance In Oxidatively Stabilized Polyacrylonitrile Subjected to Elevated Temperature", J. Appl. Phys. 52 (11) November 1981. The article indicates that resistivity measurements were made after the resistance reading was constant for at least 1 minute. While Lerner reports variations in resistivity with pyrolyzation temperature, no non-ohmic effects are noted.
Brom et al have studied the conductivity of pyrolyzed polyimide (KAPTON) film as a function of pyrolysis temperature. Brom et al, "On New Conducting Polymer-Pyrolyzed Kapton", Solid State Communications, Vol. 35, p. 135 (Pergamon, 1980). Brom et al cut the pyrolyzed film into rectangular or needle shapes. At a controlled measurement temperature of 4.2.degree. K., Brom et al report that no deviation from ohmic behavior was seen up to voltage gradient of 2.times.10.sup.3 V/cm.
Gittleman et al postulate a structure for pyrolyzed polyimides in their article, "Are Pyrolyzed Polyimides Conducting Polymers?" Journal of Electonic Materials, Vol. 10, No. 2 (1981). Gittleman et al also suggest the application of higher fields to pyrolyzed polyimide samples to test the validity of a theoretical "charging energy" model.
The voltage transient is proportioned to the rate of change of the current and the size of the inductance in the circuit. Relays and transformers have 10-100's of millihenries, while motors may have inductances of many henries. Thus, the voltage transients which occur when a fuse blows can easily be thousands of volts and can cause considerable damage to equipment. There is consequently a need for a simple and inexpensive device that, as a part of the fusing system, can decrease the rate of change of the current in the circuit.
Electrical devices, made from fiber processed under selected conditions and used under selected bias and environmental conditions, exhibit switching behavior. This switching behavior embraces abrupt changes in device resistance in response to applied voltage and negative resistance in a portion of the voltage-current domain for the device.
Acrylic fibrous material may serve as a precursor material. Such materials may be prepared by conventional techniques and may be either an acrylonitrile homopolymer or n acrylonitrile copolymer which contains at least 85 mole percent of acrylonitrile and up to 15 mole percent of one or more monovinyl units copolymerized therewith.
One example of acrylic material employed is CELIOX.TM. brand fibers, manufactured for Celanese Corporation. These fibers are formed by thermal stabilization of a continuous filament acrylonitrile copolymer yarn comprising approximately 98 mole percent of recurrent acrylonitrile units and approximately 2 mole percent of recurring methyl acrylate units.
An example of suitable acrylic homopolymer is DRALON.TM. brand fibers, a commercial polyacrylonitrile homopolymer fiber manufactured by Farbenfabrik Bayer, Leverkusen, West Germany.
These exemplary precursor materials have a number of characteristics in common. They have an electrical resistivity of greater than 10.sup.10 ohm-cm. They may be partially pyrolyzed at temperature between 500.degree. C. and 800.degree. C. to produce material having an average small signal resistivity of from about one to 10.sup.6 ohm-cm at 25.degree. C., measured at a current of less than 10 microamperes. Useful devices typically have an average small signal resistivity from 1 to 100 ohm-cm. at 25.degree. C. When pyrolyzed, the materials generally retain the same general physical dimensions of the precursor. The materials do not form a skin or core of radically different composition when subjected to stabilization and pyrolysis.
Generally, the materials, with the novel electrical properties discussed below, are made by first stabilizing a yarn made from the fibers in a controlled atmosphere at temperatures less than 500.degree. C. Typically, this stabilization step is pre-oxidation performed in air or an oxygen-enriched atmosphere. The materials are then pyrolyzed in an atmosphere essentially free of oxygen and moisture at temperatures of from 500.degree. C. to 800.degree. C. in a furnace. The yarn is cooled to room temperature in the controlled atmosphere.
Individual filaments may be mounted for electrical testing in a device consisting of a single pyrolyzed filament placed on a ceramic substrate. Metallic electrodes may be evaporated onto the filament and substrate, leaving exposed an active portion of the filament having a selected length. Electrical leads may then be attached to the evaporated electrodes. Single filament electrical switches have been fabricated with active portions varying from about 2 to 30 mils in length although an electrical switch has been made of a composite of fibers, each about 100 mils in length. The small size of the active element may be of benefit in miniaturizing circuitry employing the fibers. The resulting structure is a bipolar electrical device, whose electrical properties may be measured employing a test apparatus.
Such a test apparatus includes a function generator for applying a level biasing voltage, sine wave, or single or multiple pulses (square wave or ramp voltages) to the electrical device under test. A limiting resistor may be placed in circuit with the device and function generator to prevent overload of the device when it switches to a more highly conductive state. Current flowing through the device also flows through a current sense resistor.
A ramp voltage of selected duration and amplitude can also be applied to the device in series with the current sense resistor. Current and voltage measurements can be combined in a electronic curve tracer to provide a V-I curve for the device under test.